On your last point, just like vanilla artillery, the gun can be manually elevated and depressed by using the zeroing up/down keys (pageup/pagedown by default, I think). I'm pretty sure this is just how vehicles with artillery config work and there isn't a way to change it.

Edit:

+ issue:
The Gunner and Machinegunner seats don't seem to have pop-up icons turned on for their Get In actions, meaning you have to open the action menu to see the action. The Driver seat has the quick access icon as usual.

I wasn't able to elevate/depress the gun using zeroing keys, for some reason - can you verify it on your end?

Turns out it is actually a separate keybind under Weapons called "Gun Elevation Up"/"Down", which I happened to have bound to the same as the zeroing keys for reasons which remain unclear.

Point is, that's how you do it, and I think that's hardwired into the A3 artillery system.

Many thanks, will try it out :) I suppose that makes it direct fire capable and explains the HEAT rounds as well :)